My 1998 JD 4100 HST won't go into reverse or go forward. The shift lever between low and high now just slides back and forth and won't click into either position. I was moving some dirt around and stopped to back up and tractor wouldn't move either forwards or reverse. Tractor was just serviced by dealer and hydraulic fluid is good. Tractor has 550 hours on it. Any advice would be greatly appreciated. Thanks.